A conventional extending mechanism of the wire-rope type (see, for example, PTL 1) includes a first boom member, a second boom member that is movable with respect to the first boom member in the extension/contraction direction, a third boom member that is movable with respect to the second boom member in the extension/contraction direction, a driving part that provides a driving force for moving the second boom member with respect to the first boom member in the extending direction, an extending sheave provided at the leading end side of the second boom member, and an extending wire that is wound around the extending sheave and couples the base end side of the first boom member and the base end side of the third boom member. In this conventional extending mechanism of the wire-rope type, the third boom member is moved with respect to the second boom member in the extending direction by moving a coupling part of the extending wire wound around the extending sheave on the third boom member side in the extending direction of the boom by the operation of the second boom member that moves with respect to the first boom member in the extending direction with the driving part.
In the extending mechanism of the wire-rope type, the base end side of the first boom member and the base end side of the third boom member are coupled with each other with a plurality of extending wires. In the case where the first boom member and the third boom member are coupled with each other with a plurality of extending wires, wire terminal metal fittings are attached to both end portions of the extending wires, and the wire terminal metal fittings are coupled with the first boom member and the third boom member through the coupling metal fitting. In the case where the first boom member and the third boom member are coupled with each other with a plurality of extending wires, two wire terminal metal fittings are required for each extending wire, and consequently the manufacturing cost of the extending wire is increased.
In view of this, an extending mechanism of the wire-rope type which can reduce the manufacturing cost of an extending wire is conceivable (see, for example, PTL 2). In this extending mechanism of the wire-rope type, a plurality of extending wires are composed of one wire member, an equalizer sheave is provided to one or both of the base end side of the first boom member and the base end side of the third boom member, and the extending wire is wound around the equalizer sheave, whereby the extending wire is coupled with the first boom member and the third boom member.